1. Field of the Invention
The present invention relates to an iron-based sintered alloy material suitable for a valve seat in internal combustion engines, and particularly to an iron-based sintered alloy material having improved wear resistance and reduced opposite aggressibility to mating material.
2. Description of the Related Art
Sintered alloy material is commonly manufactured by blending, mixing and kneading the raw materials to produce a mixed powder, compressing the resulting mixture in a mold, and sintering the resulting compact at a predetermined temperature in a predetermined atmosphere. Thus, metals or alloys that are difficult to produce by common melting and solidification processes can be easily manufactured in the form of sintered alloy material. In addition, since multiple functions can be easily combined in the sintered alloy material, a component having a particular function can be easily produced. The sintered alloy material is also suitable to produce a porous material, a material having a low machinability, and a component in a complicated shape.
A valve seat is press-fit or joined to a cylinder head in an internal combustion engine, preventing leakage of combustion gas and cooling a valve. The valve seat is therefore hit by the valve, wears by friction, is heated by the combustion gas, and is exposed to corrosive combustion products. Thus, the valve seat requires heat resistance and wear resistance, and the sintered alloy material has recently been used in such a valve seat.
In view of environmental protection, automobile engines have been required to have an extended life, higher power, and improved fuel efficiency, and to emit cleaner exhaust gas. To satisfy such demands, the automobile engines are operated under severer conditions, and accordingly valve seats are also exposed to severer environments. In such environments, conventional valve seats may have insufficient heat resistance and wear resistance.
To address such problems, Japanese Unexamined Patent Application Publication No. 5-43913 discloses a valve seat composed of an iron-based sintered alloy material having reduced opposite aggressibility to mating material. This valve seat is composed of 5% to 25% by weight of spherical carbide-dispersed hard particles and/or intermetallic compound-dispersed particles, each particle having a microvickers hardness (MHV) of 500 to 1800 and being dispersed in a matrix. With such a micro structure, the valve seat is claimed to have reduced opposite aggressibility to mating material. The intermetallic compound-dispersed particles may be composed of Mo: 20% to 40%, Cr: 5% to 15%, and Si: 1% to 5%, the balance being Co and unavoidable impurities.
Japanese Unexamined Patent Application Publication No. 11-6040 proposes an wear-resistant iron-based sintered alloy in which 2% to 30% by weight of nickel-based hard particles and/or 2% to 4% by weight of intermetallic hard particles that have higher hardness than the nickel-based hard particles, such as Fe—Mo, Fe—W, or Fe—Cr, are dispersed in a matrix that is formed by mixing and sintering an Fe—Co—Mo— based alloy powder and an Fe—Cr— based alloy powder. With such a structure, this iron-based sintered alloy is claimed to have reduced opposite aggressibility to mating material and increased wear resistance.
Japanese Unexamined Patent Application Publication No. 2003-268414 also proposes a sintered alloy for a valve seat in which 1% to 3% by weight of enstatite particles, 15% to 25% by weight of hard alloy particles (A) having a HV of 500 to 900, and 5% to 15% by weight of hard alloy particles (B) having a HV of 1000 or more are dispersed in a sintered alloy skeleton matrix containing carbides of Cr, Mo, W, and V, and pores in the skeleton matrix are infiltrated with copper or a copper alloy. The structure composed of two types of hard particles having different hardness is claimed to improve the wear resistance of the valve seat and to reduce the wear loss of a mating valve. The hard alloy particles (B) may be ferromolybdenum particles or high-alloy hard particles containing tungsten.